The present invention relates to thermoforming apparatus and, in particular, to a thermoforming machine for thermoforming three thermoplastic sheets over three thermoforming molds to produce an instant a unitary triple sheet article.
The art of twin sheet thermoforming is well known and has been practiced extensively in recent years in the construction of plastic articles. Twin sheet thermoforming has at least two intended purposes which include the production of hollow articles having greater strength than similar articles formed from a single sheet of thermoplastic, and the consolidation of more than one thermoplastic sub-component into a single unitary article.
The apparatus of twin sheet thermoforming was pioneered by a handful of inventors in the late Sixties and early Seventies. A number of different types of twin sheet apparatus were developed and implemented during this period of time. Examples of these early types of twin sheet thermoforming apparatus may be known by referring to U.S. Pat. Nos. 3,398,434 to Alesi, Jr., et al., 3,583,036 to Brown, 3,597,799 to Earle, 3,779,687 to Alesi, 3,783,078 to Brodhead, 3,787,158 and 3,867,088 to Brown et al.
The basic apparatus and principles of twin sheet thermoforming preferred by practitioners, as of the time of the present invention, are disclosed in U.S. Pat. Nos. 3,868,209 to Howell and 3,925,140 to Brown. In fact, Brown Machine LLC, of Beaverton Mich., is internationally renown as the premier manufacturer of twin sheet thermoforming machinery. Several other machinery manufacturers, domestic and foreign, offer twin sheet thermoforming apparatus based upon the principles of the ""209 and ""140 inventions in the rotary machine configuration. Although other twin sheet machine configurations are offered, such as linear machine configurations, the rotary style is much preferred for its inherent short cycle time.
A wide variety of small and large articles thermoformed according to the twin sheet methodology are found in a large number of primary end markets including recreational and sporting goods, building and construction, automotive and transportation, marine, agricultural, appliance, industrial and consumer products to name a few. However, the largest end-market application to which twin sheet thermoforming apparatus has been widely applied involves industrial platforms such material handling pallets and the like.
The traditional wood pallet is an integral part of America""s distribution system, and is involved in one way or another in the movement and storage of the vast majority of goods within the economy. Wood pallets however have a large number of problems that well known and plastic pallets have been used to some advantage in recent years. While plastic pallets are attractive for a number of known reasons, they still have two significant shortcomings. Most notably, plastic pallets deform under heavy load and unless reinforced with steel, wood or the like, do not provide acceptable strength. Secondly, plastic materials are more expensive than wood materials and a relatively large amount of plastic is used to make a pallet comparable in strength to wood. Therefore, plastic pallets are considerably more expensive than wooden pallets.
One to the most successful types of plastic pallets is a twin sheet thermoformed pallet. Twin sheet pallets are generally constructed out of two thermoplastic sheets according to the principles and apparatus of ""140. An early example of a pallet formed in this general character is disclosed in U.S. Pat. No. 4,428,306 to Dresen et al. The two significant shortcomings associated with plastic pallets are not, however, overcome with twin sheet thermoforming.
Phil Araman of the United States Forest Service has estimated that there are 1.9 billion pallets within the U.S. distribution system, and that roughly 400 million new pallets are added to maintain the over-all inventory each year. Of this annual volume, twin sheet thermoformed pallets reportedly only account for roughly 1.4 million units. Therefore, the practitioners of twin sheet thermoforming have attempted in recent years to improve the art of twin sheet thermoforming to overcome the significant shortcomings noted above in order to capture a larger share of the over-all pallet market. Some of the more notable apparatus improvements may be known by referring to U.S. Pat. Nos. 5,620,715 to Hart et al., 5,800,846 to Hart, 5,975,879 and 6,086,354 to Dresen et al., and 5,658,523 and 5,843,366 to Shuert. Upon close inspection of these improvements, it will become readily apparent that the significant shortcomings have not yet been overcome in the prior art. The improved apparatus referenced has only provided incremental improvement results and the basic apparatus still yields twin sheet pallets that have relatively low load bearing strength and remain expensive compared to wooden pallets.
Although the art of twin sheet thermoforming has resulted in improved products in a wide variety of applications and end markets, a range of needs nonetheless exists to advance thermoforming methods and apparatus to overcome such specific weaknesses as have been mentioned above, and more generally, to further increase the strength, lower the costs or improve the quality of articles constructed of thermoplastic materials.
It has been suggested that three or more sheets of thermoplastic can be combined in a form analogous to a honeycomb construction in order to overcome a wide range of strength related problems, cost issues and to achieve parts consolidation benefits. Continuing with the present example of plastic pallets, in U.S. Pat. No. 5,470,641 a routine twin sheet panel structure is over and/or under laid with separate (coextensive) sheets in separate forging like operations to provide a panel structure with one or more flat face plates. In yet another example, which is understood best by referring to U.S. Pat. No. 4,348,442, a structural panel substantially equivalent to the panel structure of ""641 is disclosed. There are a number of problems that would be readily apparent to those skilled in the pertinent arts with respect to the end products associated with these disclosures. First, it will be appreciated by referring to U.S. Pat. No. 3,919,446 that the twin sheet structure of ""641, in particular, would be more economically achieved by the single sheet expansion process of ""446. Secondly, the perimeter borders of ""641 and ""442 are normally open and could collect or retain contaminants associated within material handling environments. Third, the secondary operations of fusing, screwing and bonding the separate thermoplastic sub-components together would be difficult and costly to implement. Fourth, except as may be anticipated by ""442, the approach of ""641 is not amenable for use as a pallet structure in which legs are required for the receipt of pallet handling equipment and machinery. Fifth, as is well know, and as first taught by U.S. Pat. No. 4,428,306, single walled thermoplastic legs are not strong enough to support the tremendous static loads required by industry. Sixth, it is critical to provide a wide spread and permanent interfacial connection between all of the mating surfaces of the thermoplastic sub-components forming the referenced hybrid honeycomb panel structures in order to achieve maximum load bearing and anti-delaminating strength. The assembly methods inferred and or suggested by ""641 and ""442 would not be amenable to such wide spread and permanent interfacial unions, with the result that the articles would not be optimally robust. Seventh, the introduction of aggregates, fillers, agents, adhesives, fasteners and other non-thermoplastic joinery would create problems with respect to the recycling of the reference multi-sheet panel structures at the end of their useful lives.
Therefore, although triple, even quadruple sheet thermoplastic constructions would enjoy the theoretical strength advantages associated with honeycomb sandwich constructions, the art of thermoforming has not yet been advanced to a point where at least three thermoplastic sheets could be instantly thermoformed over three shape-giving molds to provide unitary articles that overcome the problems that by way of example have now been identified.
It is, therefore, the objective of the present invention to provide a machine for thermoforming three sheets of thermoplastic over three molds to provide, in an instant manufacturing process, a unitary triple sheet thermoplastic article.
Accordingly, an object of the present invention is to provide a machine having three ovens within which three sheets of thermoplastic can be simultaneously heated to a thermoformable state. The object is accomplished by providing three ovens comprising five oven banks, with the first oven bank uniquely positioned at the load/unload station above the wheel supporting the clamp frames that hold the individual sheets forming a triple sheet article. Although the fifth oven in the present embodiment is positioned over the load/unload station characteristic of a four-station rotary machine, this aspect can be accomplished in a five or six station rotary configuration.
According to a further aspect of this object, the three ovens comprising the five oven banks each include a plurality of infrared emitters. As each of the three sheets progress through each of the three ovens means regulating emitter output are precisely controlled to ensure that the over-all amount of heat transferred to each of the three sheets is controllable and therefore adjustable according to the length of time each of the three sheets dwells in each of the three ovens. According to this aspect, each emitter of each oven bank is controlled to emit more or less heat energy relative the heat absorption characteristics of the individual sheets of thermoplastic and the varying length of time each sheet dwells within each of three ovens.
Still according to this aspect, the invention includes means adjusting the vertical position of the first oven bank relative the wheel supporting the clamp frames carrying the plastic sheets in order to prevent triple sheet articles from impacting the first oven bank as the article is indexed forward from a from station to the load/unload station.
Another object of the invention is to provide means controlling the operation of the clamp frames. According to this aspect, means are provided for a three sheet sequence in which case programmable logic control is required to ensure the clamp frames respond to proper open or close operating functions at the form station.
Still according to this aspect, pivotally opposed pin bars mounted to co-acting solenoid operated cylinders are suggested in order to enable various modes of the invention to be used. Pivotally opposed pin bars allow the formed sheets of thermoplastic at the form station to be extracted to a position above or below a sheet line associated with the path traveled by the wheel as preferred by the thermoforming practitioner.
Another object of the invention is to provide means delivering three shape-giving molds that each separately thermoform one of the three sheets of thermoplastic of the unitary article. According to this aspect the machine has a form station comprising opposed platens that travel vertically from open positions to closed positions where the three sheets of thermoplastic are sequentially thermoformed over the three molds.
According to this aspect the opposed platens are developed to support and carry three molds into positions relative for thermoforming. In this connection, associated with at least one platen is a mold shuttle system, the mold shuttle system providing means to laterally shift one of two molds into position upon the associated platen for vertical movement into positions relative for thermoforming.
According still to this aspect, means are provided to control the open and closed positions of the platens relative for thermoforming three sheets over three molds and for operation of the mold shuttle system.
Yet another object of the invention is to sequentially compress the formed sheets of thermoplastic between the molds supported upon the platens to first provide a twin sheet sub-assembly and then a unitary triple sheet article. According to this aspect the platens are closed in a manner having the attributes of a forging operation. Means for implementing the forging-like operation first between a first set of molds and next between a second set of molds are provided.
According to still yet another object, a machine frame to support the equipment associated with the apparatus is disclosed in connection with a rotary machine configuration, although a liner machine frame falls within the scope of the invention.
Other features, objects and advantages of the present invention apparatus will become apparent from the following description and appended claims when viewed in conjunction with the accompanying drawings.